Support project wide code check

[jrieken]

When changing code I often break other files that depend on the file I am currently editing, like tests. IMO it would be beneficial if OmniSharp gives me a change to check the whole project at once. I am thinking of /codecheck without a filename.

[nosami]

Sounds good to me.

[jrieken]

on a second though we can also use the event emitter to send back diagnostics whenever buffers update. that way Omnisharp has a chance to be smart and delay validation when documents constantly change. but maybe that's a different story

[nosami]

fixed by #133